An internal combustion engine usually has an air supply unit composed of an intake box (also known as an “air box”) which is in communication with the external environment by means of a respective air filter and at least one butterfly valve interposed between the intake box and the intake ducts of the cylinders in order to regulate the flow rate of the air supplied to the cylinders themselves.
According to some design philosophies for internal combustion engines, each engine cylinder is provided with a respective butterfly valve; however, this design approach is associated with some disadvantages, in that once the engine is installed in the respective vehicle, it is extremely difficult to gain access to the individual butterfly valves in order to regulate them, both during vehicle assembly and during periodic maintenance of the vehicle. Moreover, providing each cylinder with a respective butterfly valve brings about an obvious increase in the cost and complexity of the air supply unit because of the greater number of components in the air supply unit itself.
EP0501514A1 discloses an intake system for a multi-cylinder type internal combustion engine comprising throttle valves disposed in a plurality of intake passages commonly connected to an air cleaner, and fuel injection valves disposed in the intake passages downstream of the throttle valves in an intake direction. In such intake system, a plurality of funnel-shaped portions are provided in a throttle body having opposite ends directly coupled to an intake manifold and an air cleaner, respectively, and each of the throttle valves is disposed in the throttle body, so that its upstream end in the intake direction is located at least on the corresponding funnel-shaped portion, when the throttle valve is fully opened; the throttle body includes ports provided therein, which are opened into the intake passages between a coupled surface of the throttle body to the intake manifold and the throttle valves, respectively, and which communicate with one another.